pH and Orthophosphate

DC Water's Corrosion Control Performance

In addition to lead testing, DC Water tests the water for other parameters
that could indicate the performance of the corrosion control treatment. Two
key parameters are pH and orthophosphate.

Water becomes corrosive at lower pH values (<7.0 pH units). In addition,
the corrosion treatment chemical added to the water (orthophosphate) works
best at a pH range of 7.0 to 8.0. The Washington Aqueduct, who treats the
water, is required to maintain a pH leaving the plant between 7.4 and 8.0.
DC Water monitors for pH in the distribution system daily to ensure these levels
stay within the optimum range for orthophosphate. DC Water also set a more
stringent range of 7.4 to 7.8 to target in the distribution system.

During the initial phase of the corrosion control treatment experts recommended
a higher dose of orthophosphate — 3.5 mg/L. Once the orthophosphate formed a
protective layer on the pipe scales, the dose was reduced to 2.5 mg/L leaving
the plant. DC Water set a range of 1.0 to 4.5 mg/L orthophosphate to target
in the distribution system.

orthophosphate results

The graphs below shows the maximum, average, and minimum results for pH on
a monthly basis and orthophosphate on a quarterly basis, over the past year
collected at compliance monitoring sites in DC Water's distribution system.
The top and bottom of the lines represents the maximum and minimum values.
The black dash in the middle of the line represents the average value.